1. Field of the Invention
The present invention relates to a method of maintenance for an ink jet head in a case where ink of each color is spouted from a plurality of ink jet heads to thereby form an image on an image forming medium, and an image forming apparatus using this method.
2. Description of the Related Art
As shown in FIG. 16, a plurality of ink jet heads 1 are used in an ink jet system image forming apparatus. Each ink jet head 1 has a nozzle plate 3 and piezoelectric elements (PZT) 4. A plurality of nozzles 2 are formed in the nozzle plate 3. A piezoelectric element 4 is disposed for each of the nozzles 2. Each ink jet head 1 has an ink chamber 5 formed by the piezoelectric element 4 for each nozzle 2.
Respective ink jet heads 1 are disposed for the colors of black (K), cyan (C), magenta (M), and yellow (Y). Each ink chamber 5 of the ink jet head 1 is charged with each color of ink 6. When each piezoelectric element 4 is displaced/operated at an image forming time, the pressure in each ink chamber 5 rises. The ink 6 is spouted from each nozzle 2.
When a back pressure is applied to each ink chamber 5 in the ink jet head 1, a meniscus 7 is formed in each nozzle 2.
Additionally, when dust, dirt, air, degraded ink liquid and the like are mixed in the ink chamber 5, and deposits such as dust and dirt adhere to the nozzle 2 or the vicinity of the nozzle 2, the meniscus 7 collapses. Therefore, the amount of each ink 6 spouted from the nozzle 2 is not appropriate. A jet direction of the ink 6 changes. As a result, a high-quality image cannot be formed.
Therefore, maintenance for the ink jet head 1 is performed. For example, the following technique is described in Jpn. Pat. Appln. KOKAI Publication No. 2002-347260.
A predetermined nozzle in the ink jet head is sealed by sealing means such as a suction cap. The inside of the sealing means is sealed, and brought into a negative pressure state. In this state, the sealing means including the suction cap or the like is swept/moved with respect to the ink jet head. The sealing means including the suction cap sucks remaining ink in the nozzle. Accordingly, bubbles or foreign matter in the nozzles is discharged.
In the maintenance for the ink jet heads 1, the bubbles, foreign matter and the like in the nozzles 2 and the ink chambers 5 are discharged, collapsed meniscuses 7 are recovered, and generation of newly collapsed meniscuses 7 is prevented. When the sealing means brought into the negative pressure state is simply swept and moved as in the Jpn. Pat. Appln. KOKAI Publication No. 2002-347260, it is difficult to securely recover the meniscuses 7 in the nozzles 2 of the ink jet heads 1.
To solve the problem, in a constitution shown in FIG. 17, maintenance for ink jet heads 1k-1 to 1y-6 is performed for each color (K, C, M, Y).
The plurality of ink jet heads 1k-1 to 1y-6 are connected to distributors 10-1 to 10-4 for each color (K, C, M, Y). The respective distributors 10-1 to 10-4 are connected to ink tanks (sub-tanks) 12-1 to 12-4 via tubes 11-1 to 11-4.
The respective ink tanks 12-1 to 12-4 are charged with inks 6k, 6c, 6m, 6y of the respective colors. The respective ink tanks 12-1 to 12-4 are connected to atmosphere open tubes 13-1 to 13-4. The respective atmosphere open tubes 13-1 to 13-4 are provided with atmosphere open valves 14-1 to 14-4. It is to be noted that liquid level height positions of the respective inks 6k, 6c, 6m, 6y in the ink tanks 12-1 to 12-4, and height positions of the respective nozzles 2 of the ink jet heads 1k-1 to 1y-6 are set to a difference of elevation optimum for applying a back pressure to the respective ink chambers 5 of the ink jet heads 1k-1 to 1y-6 to thereby form the meniscuses 7 in the nozzles 2.
The ink tanks 12-1 to 12-4 are connected to air tanks 16-1 to 16-4 via communication tubes 15-1 to 15-4. The respective communication tubes 15-1 to 15-4 are provided with pressurizing valves 17-1 to 17-4. The ink tanks 12-1 to 12-4 are connected to ink bottles (not shown) of the respective colors. The inks 6k, 6c, 6m, 6y of the respective colors are supplied to the ink tanks 12-1 to 12-4 from the respective ink bottles.
The respective air tanks 16-1 to 16-4 are connected to pressurizing pumps 19-1 to 19-4 via pressurizing tubes 18-1 to 18-4.
Cleaning members 20k-1, 20k-2, 20k-6 are disposed for the ink jet heads 1k-1 to 1y-6. In FIG. 17, the only cleaning members 20k-1 to 20k-6 for the color K are shown in order to avoid laborious drawing.
The respective cleaning members 20k-1 to 20k-6 are connected to a suction pump 22 via a suction tube 21. The respective cleaning members (suction heads) 20k-1 to 20k-6 and suction nozzles for the other colors slide with respect to the ink jet heads 1k-1 to 1y-6.
Next, a maintenance operation will be described.
First, the respective atmosphere open valves 14-1 to 14-4 close. Moreover, the pressurizing valves 17-1 to 17-4 close.
Next, the respective pressurizing pumps 19-1 to 19-4 are driven. Air from the pressurizing pumps 19-1 to 19-4 is supplied to the air tanks 16-1 to 16-4 through the pressurizing tubes 18-1 to 18-4. At this time, since the pressurizing valves 17-1 to 17-4 are closed, pressures in the air tanks 16-1 to 16-4 rise.
When the pressures in the air tanks 16-1 to 16-4 reach predetermined pressures (pressures required for purging), the pressurizing valves 17-1 to 17-4 are opened. Accordingly, the purge pressures in the air tanks 16-1 to 16-4 are applied to the ink tanks 12-1 to 12-4 through the pressurizing valves 17-1 to 17-4. The pressures in the ink tanks 12-1 to 12-4 rise.
By the rising of the pressures, the respective inks 6k, 6c, 6m, 6y of the colors stored in the ink tanks 12-1 to 12-4 are supplied to the distributors 10-1 to 10-4 via the tubes 11-1 to 11-4. Furthermore, the inks 6k, 6c, 6m, 6y are supplied to the ink jet heads 1k-1 to 1y-6.
Accordingly, the respective inks 6k, 6c, 6m, 6y are discharged from the nozzles 2 of the ink jet heads 1k-1 to 1y-6. When the inks 6k, 6c, 6m, 6y are discharged, bubbles, foreign matter and the like in the nozzles 2 and the ink chambers 5 are discharged. The discharging of the bubbles, foreign matters and the like in the nozzles 2 and ink chambers 5 by the discharging of the inks 6k, 6c, 6m, 6y is referred to as purging.
Next, the respective atmosphere open valves 14-1 to 14-4 are opened. Accordingly, the pressures in the ink tanks 12-1 to 12-4 are reduced. At this time, when open/close timings of the atmosphere open valves 14-1 to 14-4 are controlled, pressure reduction in the ink tanks 12-1 to 12-4 is controlled. By this control, the pressures in the ink chambers 5 of the ink jet heads 1k-1 to 1y-6 are set to predetermined micro-positive pressures.
By the setting of the micro-positive pressures, the inks 6k, 6c, 6m, 6y are gradually discharged from the nozzles 2 of the ink jet heads 1k-1 to 1y-6. It is to be noted that even when the respective inks 6k, 6c, 6m, 6y are gradually discharged from the nozzles 2, the ink remains to such an extent that the ink does not drop onto the nozzle plate. This state is maintained before/after a sucking operation described later.
In this state, the respective cleaning members 20k-1 to 20k-6 are brought into contact with the nozzle plates 3 of the ink jet heads 1k-1 to 1y-6, and slid. Accordingly, the cleaning members 20k-1 to 20k-6 scrape and suck the inks 6k, 6c, 6m, 6y discharged to the nozzle plates 3 of the ink jet heads 1k-1 to 1y-6. Accordingly, the cleaning members 20k-1 to 20k-6 remove deposits adhering to the nozzle plates 3. To move the cleaning members 20k-1 to 20k-6 while removing the deposits from the nozzle plates 3 will be referred to as sucking.
Next, the atmosphere open valves 14-1 to 14-4 are opened. When the atmosphere open valves 14-1 to 14-4 are left in the open state, back pressures are applied in the ink chambers 5 of the ink jet heads 1k-1 to 1y-6. The meniscuses 7 are formed in the nozzles 2 of the ink jet heads 1k-1 to 1y-6.